Planar Microstrip Trisection Filters

Abstract

Bandpass filters are the key components of the wireless communication system. Modern applications of wireless communication need filters with low insertion loss, compact size, and high selectivity. Traditional approaches for achieving selectivity like increasing the number of the resonator is not a good option as both size and insertion loss is compromised. In order to meet the demanding requirement cross coupling based approach is used which generates the transmission zero (TZ) for improving the filter out of band rejection along with size reduction. This thesis presents the design, simulation, and fabrication of microstrip trisection filter based on two different topologies. In one topology a trisection filter was designed using open-loop resonators. This filter exhibits a center frequency of 954.95 MHz, a return loss of 11.1 dB, 3dB bandwidth of 46.3 MHz, and an insertion loss of 1.02 dB. The response of this filter is asymmetric, and the transmission zero in this case is on the right side of the passband. Due to the sharp roll off out of band rejection of -30 dB was achieved at 993 MHz. Another planar microstrip trisection filter was designed at a center frequency of 775 MHz. This filter produces a single or multiple transmission zeros by changing the tap position on the resonators. It is shown that two responses namely asymmetric response (transmission zero on either the right or left side of the passband) or symmetric response (transmission zeros on both sides of the passband) can be realized using this method. The measured results are close to simulated ones.